During drilling and casing of wells, a pipe string is assembled by adding stands of pipe, consisting of one or a plurality of pipe sections, to pipe strings. The pipe strings extend through the drilling floor rotary opening and continue downward into existing well bores. The pipe string being assembled may be drill strings or casing strings, and occasionally other tubular strings.
The rate of the pipe string assembly is part of the well drilling time involvement and can amount to many hours of total well producing time involved. Cost reduction, involves time reduction in pipe string assembly.
Well production time, in terms of drilling rate, has been addressed with great earnest for many years. Pipe string assembly rate has about the same cost effectiveness as drilling rate. This invention addresses the reduction of costs, and does so within the safety concerns common to well bore production and production expected of completed wells.
For safety reasons, the use of personnel in contact with tubulars during pipe string assembly on the drilling floor is being minimized. Full mechanization of such activities on the drilling floor is not always possible but every effort to limit the contact between the more dangerous activities and people is worthwhile.
Offshore drilling rigs are usually massively complex and costly and the addition of machinery approaching automatic functions is not an expense that is a large percentage of the overall costs. On smaller on-shore rigs, the complex machinery is not readily adaptable and the reduction of contact between men and machinery is approached with simpler apparatus such as the present invention.
Pipe being assembled by adding threaded sections is rotated at the upper end during the thread run. At times, during the lowering of the pipe string into the well, the pipe string is rotated to facilitate installation in the well and that is done from the top of the string.
In economic interest, the feed rate during the lowering of the string into the well is maximized, within the limits of safety considerations. The downwardly moving string occasionally encounters cause for brief stoppage, usually called ledging. The massive hoisting machinery that supports the moving pipe string is hard to stop and the result is usually a jarring experience in many respects. There is a need for some form of shock minimizing apparatus between the pipe string and the hoisting machinery. The cushioning effect is often called float and one feature of the present invention is to provide that enhancement.
Machines carrying heavy and dangerous loads normally have a design safety margin that seems adequate unless shock is encountered. Shock that is inherent to function, such as ordnance, has been defined through tests but such tests are not possible in oil field hoisting situations. Any potentially dangerous shock load needs a cushioning factor if it is not well defined and considered during design.
The expression “wedge lock gripper” usually refers to the inclusion of a self-locking taper that will not “un-do” if the actuating load is removed. To make the wedge lock feature “fail safe” the supported load needs to urge the wedge part engaging the load in the grip actuating direction. Increasing load then increases the grip. The gripping dies and the activating wedge is often called a grapple.
Once a maximum load is realized, and the fail safe feature has increased the grip on the load, the force needed to disengage the wedge lock could well exceed the force used to actuate the fail safe system. Reserve force may be needed to unlock the pipe gripper when appropriate. The wedge-lock feature of fail-safe apparatus is seldom, if ever, released during the time that the pipe string is supported by the main hoist apparatus. A forceful release feature related to the wedge-lock assembly is suitable if it can release only one stand of pipe. The forceful release, however, needs to release the wedge-lock assembly that has been set by the massive load of a pipe string. The feature providing the float quality to the pipe string, if not arranged to force the release, needs to be augmented by structure that can provide the force needed to release a heavily engaged wedge-lock assembly. The present invention is, in part, directed toward the certainty of wedge-lock release ability.
If a pipe string can be deliberately released by the supporting means, sooner or later it will be done accidently. It is prudent to provide an arrangement that will hold a pipe string in suspension regardless of any effort made to release it. The apparatus of this invention addresses that need.